1. Field of the Invention
This invention relates to electrochemically producing alkali metal stannates and, in particular, to an electrochemical process for manufacturing potassium stannate and sodium stannate.
2. Description of the Prior Art
Traditionally, alkali metal stannates have been produced by the chemical reaction of tin hydroxide with an appropriate alkali metal hydroxide or by chemically or electrochemically producing stannic (quadrivalent) tin with the appropriate alkali metal hydroxide to produce the desired alkali metal stannate. These known processes, however, pose certain insurmountable problems that predicate the need for an improved process for producing needed alkali metal stannates for use in a variety of commercial applications, including metal plating.
In the first of the above-mentioned processes, the production of the alkali metal stannate generally proceeds by chemically reacting tin hydroxide in solution with an excess of the appropriate alkali metal hydroxide in a heated reaction vessel. When the desired reaction is completed, the solution is cooled and filtered, and the alkali metal stannate is recovered by crystallization. However, in recent years, the amount of available tin hydroxide usable for this method of manufacturing has diminished drastically; and it is, therefore, impractical or not possible to use this method on a large scale to produce the required quantities of alkali metal stannates that are in commercial demand.
In either of the latter of the above-mentioned processes, other difficulties are posed by the inherent necessity of producing stannic (quadrivalent) tin by chemical reaction with the appropriate alkali metal hydroxide to produce the desired alkali metal stannate. When proceeding chemically, the process involves reacting tin with an appropriate alkali metal hydroxide in the presence of an oxidizing agent, such as sodium nitrate, sodium nitrite or both. An oxidizing agent is necessary in this process as tin will not dissolve readily in alkali solutions. Moreover, the addition of a substantial amount of heat over a considerable amount of time is necessitated in order to promote oxidation and reaction. Also, it is necessary for practical reasons to keep the solution relatively dilute and to maintain an excess of both the alkali metal hydroxide and the oxidizing agent in the reaction solution.
The reaction proceeds according to the following general equation: EQU Sn + NaOH + NaNO.sub.3 + 4H.sub.2 O.fwdarw. Na.sub.2 Sn (OH).sub.6 + NH.sub.3 + O.sub.2
when the desired reaction is completed, the dilute solution must then be concentrated at the expense of the use of a substantial additional amount of heat; and the alkali metal stannate is thereafter recovered by crystallization. In such a process, it will be apparent that there is required the expenditure of both an oxidizing agent and a substantial amount of heat for the oxidation of the tin for reaction and for concentration of the solution. Moreover, the process requires for its effective utilization either the use of scrap tin plate, which is becoming increasingly scarce, or prilled, mossy or otherwise finely comminuted tin. The reason for the use of scrap tin plate or prilled, mossy or otherwise finely comminuted tin is that, even with an oxidizing agent, the rate of dissolution of the tin is very slow and large surface area is, therefore, needed in order for the reaction to occur at any commercially acceptable rate.
Similarly, it is known to produce alkali metal stannates electrochemically by anodically dissolving stannic (quadrivalent) tin into an alkali metal hydroxide such that by direct reaction an alkali metal stannate is produced. However, such a process is extremely sensitive. It requires very precise control in order to prevent the tin from again plating out. Precise control of the oxygen necessary for filming the anode is also required so as to avoid forming an oxide film on the anode which will cause the anode to stop dissolving. Moreover, as will be seen hereinafter, the electrical power required in such a process is substantial, thereby not only increasing the cost of the end product but also utilizing substantial energy at the present time when energy conservation is of such great importance.